This invention concerns formulations for use in the generation of phosphine. These formulations are particularly suited for the production of phosphine for use in the fumigation of grain and similar particulate commodities stored in bulk, but they are not limited to that purpose.
Phosphine (PH3) has been a preferred gaseous fumigant for stored grain and similar particulate commodities because any residue of the fumigant will be lost or oxidised to a harmless phosphate when the grain or other commodity is processed to produce a food. Examples of the fumigation of grain with phosphine are found in the specifications of: International patent application No. PCT/AU90/00268 (which is WIPO Publication No. WO 91/00017); U.S. Pat. Nos. 4,059,048, 4,200,657, 4,756,117 and 4,812,291; and Australian patent No.598,646 and its equivalent South African patent No. 86/4806. This list is not exhaustive.
The phosphine used to fumigate a grain silo or other storage may be supplied by cylinders which contain phosphine or (more usually) a mixture of phosphine and carbon dioxide. Alternatively, the phosphine may be supplied by on-site generators in which phosphine is produced by the reaction of water or water vapour with a metal phosphide (typically aluminium phosphide or magnesium phosphide). However, the more traditional technique for disinfecting a grain silo or the like with phosphine involves the use of a probe to position formulations of aluminium phosphide or magnesium phosphide within the grain mass, where moisture in the air circulating through the grain reacts with the phosphide in the formulation to generate phosphine.
A modification of this technique involves the placement of pellets of a metallic phosphide in the head space of a grain silo. The phosphine produced when the phosphide reacts with moisture in the atmosphere above the grain is circulated through the stored grain either by the natural convection currents that are present in the silo or by active recirculation of air from the head space through the grain using recirculation ducts. Examples of this approach are found in the aforementioned specifications of U.S. Pat. Nos. 4,200,657 (to J S Cook) and 4,756,117 (to W. Friemel). Such techniques, however, cannot be guaranteed to supply phosphine to all regions of the bulk stored grain to eradiate weevils and other unwanted grain pests. In addition, when these traditional techniques are used, there is no control over the concentration of phosphine in the grain after the phosphide formulation has been added, and there is no possibility of regulating the phosphine concentration/time regime in the bulk storage to be most effective.
When using phosphine from cylinders which contain this gas (they usually contain 2 per cent phosphine and 98 per cent carbon dioxide), the phosphine should be mixed with air externally of the stored grain and the resultant gas mixture should then be pumped through the grain. This method of disinfestation of the grain should be carried out periodically, or whenever it is perceived that fumigation of the grain is necessary or desirable. Although this approach to the fumigation of grain is regarded as the most efficient, its major disadvantage is the need to guarantee a supply of expensive cylinders of the gas mixture to the treatment site.
There is one significant problem with the use of phosphine. When its concentration in air reaches certain values, the gas mixture is flammable and potentially explosive. The generally accepted maximum safe concentration of phosphine in air at standard temperature and pressure is 1.79 per cent by volume. The paper entitled xe2x80x9cThe Flammability Limit of Pure Phosphine-Air Mixtures at Atmospheric Pressurexe2x80x9d, by A. R. Green et. al., which was published in xe2x80x9cControlled atmosphere and fumigation in grain storagesxe2x80x9d (Edited by B. E. Ripp et. al.), Elsevier, 1983, pages 433-449, confirms this safe concentration limit and provides further information about the flammability of phosphine.
Other potential problems with the use of phosphine are the toxicity of phosphine to mammals when the phosphine is present in high concentrations, and the exothermic nature of the hydrolysis reaction which produces the phosphine.
Thus the use of on-site phosphine generators, such as the generator described in European Patent Publication No. A-0318040 (the specification of European patent application No. 88119701.6), presents a number of hazards to the user. In addition, with the fumigation regimes now being employed (for example, maintaining low phosphine concentrations for long periods), it is possible that the on-site phosphine generators may be left unattended for a considerable time. This is particularly likely to be the situation when the phosphine generator is used for the fumigation of storages at remote locations. Thus on-site phosphine generators, with their attendant fire or explosion risk, have been unattractive for long term fumigation operations.
It is an objective of the present invention to provide formulations of a reactive phosphide which are inherently safe for storage and/or transportation and which can be added periodically to water, or over which moist air can be passed, so that phosphine is generated in a controlled manner. In achieving this objective, the present invention provides an alternative to the phosphine generation systems (which use either phosphides in powder form positioned within a chamber closed by a membrane which is porous to moisture and phosphine, or the periodic dropping of pellets of phosphide into a water bath) which are described in the specifications of International patent application No. PCT/AU91/00264 (which is WIPO publication No. WO 91/19671) and its equivalent U.S. Pat. No. 5,573,740.
The present invention achieves this objective by mixing together a particulate phosphide and a water-immiscible compound which is inert with respect to the phosphide. The water-immiscible compound may be petroleum jelly or a non-volatile oil (for example, paraffin oil), or a mixture of petroleum jelly and a non-volatile oil. Preferably, however, the water-immiscible compound is a wax which has a melting point in the range of from about 35xc2x0 C. to about 65xc2x0 C.
If the water-immiscible compound is petroleum jelly and/or a non volatile oil, the quantity of water-immiscible compound in the formulation must be sufficient to ensure that the formulation is capable of being extruded through an orifice (for example, for feeding into a water bath) at the temperature of use of the formulation.
In some uses of the formulation, moisture-laden warm air will be passed over a sample of such a formulation. Thus a sample of the formulation must slump in a controlled manner (to expose the phosphide particles in the formulation) at its temperature of use. These constraints mean that the particulate phosphide must comprise from about 65 per cent (by weight) to about 85 per cent (by weight) of the formulation, and is preferably in the range of from 70 to 75 per cent (by weight) of the formulation. These phosphide concentrations have also been found to be appropriate when the water-immiscible compound of the formulation is a wax having a melting point in the specified range.
When a formulation in accordance with this invention, having petroleum jelly and/or a non-volatile oil as its water-immiscible compound, is placed in a water bath, or in a moist air flow, the covering of the phosphide particles (which may be pellets) is at least partially dispersed. Water, in liquid or vapour form, then comes into contact with the exposed phosphide, and reacts with it to form phosphine and a hydroxide.
If, however, a sample of the formulation which has a wax as its water-immiscible compound is placed in a water bath or in a flow of moist gas (moist air),
(a) if the formulation sample has exposed phosphide particles at its surface, the water (in liquid or vapour form) comes into contact with the exposed phosphide and reacts with it to form phosphine and a hydroxide; but
(b) if there is no exposed phosphide particle at the surface of the formulation sample (for example, because the sample was given a thin coating of the wax used in the formulation), the water, in liquid or vapour form, must first expose the phosphide particles near the surface of the formulation by melting the wax (or the surface of the formulation has to be scratched or a shaving has to be removed from it) to allow the water to contact and react with the exposed phosphide and form phosphine and a hydroxide.
Thus, according to the present invention, there is provided a phosphide formulation for use in the controlled production of phosphine, the formulation comprising a mixture of:
(a) a particulate phosphide which reacts with water to form phosphine; and
(b) a water-immiscible compound which is inert with respect to the phosphide, said water-immiscible compound being selected from the group consisting of (i) petroleum jelly; (ii) a non-volatile oil; (iii) a mixture of a non-volatile oil and petroleum jelly; and (iv) a wax having a melting point in the range of from about 35xc2x0 C. to about 65xc2x0 C.;
the particulate phosphide in the formulation comprising from about 65 per cent (by weight) to about 85 per cent (by weight) of the formulation.
Any suitable phosphide may be used in this formulation. The specifications of aforementioned Australian patent No. 598,646 and South African patent No. 86/4806 state that phosphine for fumigation of grain silos and similar bulk storage structures can be generated by xe2x80x9cthe hydrolysis of phosphides, such as those of calcium, magnesium, aluminium, zinc and tin . . . xe2x80x9d. In practice, a phosphide that is suitable for inclusion in the formulation will be selected from materials that are available at a reasonable cost.
Thus, the person who xe2x80x9cperforms the inventionxe2x80x9d (that is, the person who makes up the formulation for the controlled generation of phosphine) will select a phosphide from the materials that are available, bearing in mind the manner in which the invention will be used. For example, when the formulation is to be used in Australia for the fumigation of grain (this is the main area of interest to the present inventors), aluminium phosphide or magnesium phosphide will be used in the formulations because these phosphides (i) are readily available in Australia, (ii) are relatively low priced phosphides, and (iii) do not leave unacceptable residues. If the phosphine is to be produced slowly, aluminium phosphide would be preferred since magnesium phosphide reacts with water more rapidly than aluminium phosphide. However, in China, calcium phosphide may be preferred, for it is understood by the present inventor that calcium phosphide is more commonly available in that country. Zinc phosphide may be used only if the phosphine is produced at a slow rate, or in conditions that ensure that a significant concentration of diphosphine (which is also produced when zinc phosphide reacts with water, and which is spontaneously flammable) is not generated.
In view of the country of origin of this invention, the examples in the description of the invention refer only to aluminium phosphide or magnesium phosphide in the formulations.
The size of the particles of the phosphide in the formulation is not critical. In the initial experimentation with formulations in which the water-immiscible compound was petroleum jelly and/or a non volatile oil, to prove the efficacy of this invention, the present inventors limited the phosphide particles to a maximum diameter of 250 xcexcm by sieving, because they found this to be a useful size limitation when producing a formulation having an extrudable paste consistency. However, they quickly found that technical grade aluminium and magnesium phosphides, which have a significant proportion of particles with a diameter in excess of 250 xcexcm, can be used successfully in the formulations. Indeed, it has been found that, for the rapid (but controlled) production of phosphine over a significant period of time, small pellets of a phosphide can be used in the formulation.
If the water-immiscible compound of the formulation is a wax which has a melting point in the range of from about 35xc2x0 C. to about 65xc2x0 C. (such a wax, in this specification, will be termed a xe2x80x9clow melting point waxxe2x80x9d), the formulation will be a solid at ambient temperatures. When a thin coating of the same wax is applied to a block of this formulation, it is particularly suitable for transportation because the phosphide in the formulation is unaffected by moisture which may come into contact with the wax coating in the normal course of handling. Another benefit of this formulation, in which phosphide particles are located within a matrix of low melting point wax, is that it can readily be made into pellets. Such pellets can be used in the same manner as pellets of other phosphide formulations have been used previously (see, for example, the aforementioned specifications of International patent application No. PCT/AU91/00264 and U.S. Pat. No. 5,573,740). This is because, as noted previously, if the surfaces of the pellets contain exposed aluminium phosphide particles, water vapour (or water if the pellets are dropped into a water bath) can react with the exposed particles to generate phosphine and leave an insoluble hydroxide. When this occurs , there is a small volume increase of the reacted particles. This volume increase at the surface of a pellet distorts the surface and effectively provides the water vapour (or liquid) with access to the phosphide particle or particles directly xe2x80x9cunderneathxe2x80x9d the surface particle. This effect occurs progressively through the pellet, thus ensuring a controlled release of phosphine from each pellet.
A particular advantage of this expansion as a particle is converted from the phosphide to the hydroxide is that it makes it possible to select a surface area of the pelletsxe2x80x94or of a block of the formulationxe2x80x94according to the required rate of phosphine production. The greater the surface area, the higher the rate of phosphine production. A secondary benefit is that it is not necessary to heat the formulation to melt the wax to expose phosphide particles in the formulation. However, it should be noted that there are circumstances when it will be necessary to have the temperature of use of the formulation high enough to melt the wax. One such circumstance is when, because the formulation has to be stored for some time before the pellets (or shavings from a block of the formulation, or one or more blocks of the formulation) are dropped into a hot water bath to generate phosphine, a thin coating of wax has been applied to the formulation to protect it from possible deterioration due to the action of ambient air moisture during the storage period.
Thus it may be necessary to heat a formulation in accordance with the present invention which has a low melting point wax as its water-immiscible compound to soften its surface wax prior to its dispersion into a water bath, and/or it may be necessary to heat the water bath to a temperature which is high enough to maintain a softened state of the formulation and prevent is re-solidification, to ensure that the wax separates from the solid phosphide particles near the surface of the formulation, so that they come into contact with the water.
Any suitable low melting point wax may be used as the water-immiscible compound of the present invention. The selection of an appropriate wax will depend upon the availability and cost of the waxes from which a choice may be made.
The present inventors have found paraffin wax to be a convenient low melting point wax. Paraffin wax is a by-product of oil refining. It has a crystalline wax structure with oil held in suspension in the wax. The more oil in suspension, the lower the melting point of the wax. The present inventors have used paraffin waxes with melting points of 60xc2x0 C., 55xc2x0 C. and lower. Alternative waxes include candle waxes and thermostat waxes (such as those marketed in the U.S.A. by Astor Corporation) which have melting points within the range required for the present invention, polyester wax (which has a melting point of 37xc2x0 C. and which was invented by Dr. A. F. Steedman of the University of Glascow), certain halocarbon waxes (such as the xe2x80x9c600xe2x80x9d wax marketed by Halocarbon Products Corporation of New Jersey, U.S.A.), and a range of embedding waxes used in histology and microscopy (for example, the xe2x80x9cTissue-Tekxe2x80x9d, xe2x80x9cParamatxe2x80x9d and xe2x80x9cParaplastxe2x80x9d waxes). Note: xe2x80x9cTissue-Tekxe2x80x9d, xe2x80x9cParamatxe2x80x9d and xe2x80x9cParaplastxe2x80x9d are trade marks. This list of waxes that may be used in the present invention is not exhaustive.
If the formulation of the present invention is to be introduced into a water bath to produce phosphine, an optional additive to the formulation is a surfactant, frothicant, wetting agent, or other compound which assists in the dispersal of the water-immiscible compound in the water bath, but which is inert with respect to the phosphide component of the formulation. Dispersal of the formulation in the water of a water bath may also be assisted by the addition of a detergent to that water.
It will be apparent from the foregoing that the formulations of the present invention may be used in phosphine generators in which a formulations as described above is controllably discharged from a container into a water bath, using a pump, screw feed (auger feed) or other suitable means.
Thus, the present invention encompasses a method of generating phosphine in a controlled manner which comprises causing a small quantity of a phosphide formulation in accordance with the present invention to be introduced periodically into a water bath containing water at a temperature such that the water-immiscible compound is removed progressively from the formulation so that the phosphide particles thereof are contacted by the water of the water bath, and react with the water of the water bath to produce phosphine.
It will be necessary, sometimes, to store a formulation made in accordance with the present invention, or to transport a formulation to a grain silo or other storage structure in which the formulation is to be used. For these purposes, the phosphide formulation of the present invention may be contained in a sealed package (to ensure safe transportation and storage of the formulation). When the formulation is to be used to generate phosphine, the sealed package is opened or partially opened to expose the formulation to moist air or to another carrier gas which contains moisture (if necessary, at a temperature which causes the water-immiscible compound of the formulation to flow). The water in the air or carrier gas then reacts with exposed phosphide particles in the formulation to produce phosphine. Using this technique, the phosphine is produced in a predicable manner, the rate of phosphine production being controlled by regulating the exposed surface of the formulation, the composition of the formulation, and the water content and temperature of the air or other carrier gas.
Such a xe2x80x9cpackagedxe2x80x9d formulation can be included in existing ducting to a grain silo or other fumigation enclosure. Alternatively, it can be dropped into a water bath, to produce phosphine which is fed into ducting connected to a grain silo or other enclosure.
Another way of using the packaged formulation is to place it within a fumigation enclosure, so that moisture in the air within the enclosure can react with the phosphide in the formulation to generate phosphine.
Thus the present invention also encompasses a formulation as defined above, packaged within a moisture-impervious container, said container having at least one aperture therein, the (or each) aperture being closed by a moisture-impervious closure member, the (or each) closure member being removable to expose the formulation packed in the container.
Still further according to the present invention, there is provided a method of generating phosphine which comprises the steps of
(a) placing a packaged phosphide formulation, as recited in the last preceding paragraph, within a duct, chamber or the like through which a gas can flow, the removable region (or one of the removable regions) of the container being subsequently or previously removed from the container in which the formulation is packed; and
(b) causing a moisture-containing carrier gas to flow through said duct, chamber or the like and past the exposed phosphide formulation, whereby the water in the carrier gas reacts with the phosphide in the formulation to form phosphine, which is incorporated into the carrier gas.
Examples of formulations in accordance with the present invention and their properties, and embodiments of the methods of the present invention, will now be described, by way of example, with reference to the accompanying drawings.